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United States Patent |
5,589,969
|
Taga
,   et al.
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December 31, 1996
|
Wavelength division multiplexed optical fiber transmission equipment
Abstract
The present invention provides a wavelength division multiplexed optical
fiber transmission equipment which can increase the number of channels of
the WDM signal by reducing a required bandwidth of optical signals while
suppressing an interference between signals caused by four-wave mixing. A
WDM optical transmitting terminal comprises five or more optical
transmitters and the wavelengths of optical signals from the optical
transmitters are set such that the spacing between the wavelengths of two
signals is re-used as the spacing between the wavelengths of other two
signals separated by the above signals by two or more waves and there is
no periodicity in the channel spacing s. For instance, the wavelengths of
the optical signals are set as shown in FIG. 1. As the result,
deterioration in the transmission characteristics of the optical signals
can be effectively reduced and, at the same time, the maximum number of
channels of the WDM signal can be increased. To the WDM optical
transmitting terminal is added polarization state controllers for
controlling the state of polarization such that the states of
polarizations of adjacent signals cross each other at an output of the WDM
optical transmitting terminal.
Inventors:
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Taga; Hidenori (Saitama, JP);
Edagawa; Noboru (Tokyo, JP);
Takeda; Noriyuki (Saitama-ken, JP);
Yamamoto; Shu (Saitama-ken, JP);
Akiba; Shigeyuki (Tokyo, JP)
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Assignee:
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Kokusai Denshin Denwa Kabushiki Kaisha (Tokyo, JP)
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Appl. No.:
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542058 |
Filed:
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October 12, 1995 |
Foreign Application Priority Data
Current U.S. Class: |
398/91; 398/65; 398/79; 398/152 |
Intern'l Class: |
H04J 014/02 |
Field of Search: |
359/124,133,122,156,161
370/6,19,70,121
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References Cited
U.S. Patent Documents
5305134 | Apr., 1994 | Tsushima et al. | 359/133.
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Foreign Patent Documents |
2281670 | Mar., 1995 | GB | 359/124.
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Other References
"Reduction of Four-Wave Mixing Crosstalk in WDM Systems Using Unequally
Spaced Channels'", IEEE Photonics Technology Letters, vol. 6, No. 6, Jun.
1994, pp. 754-756 by F. Forghieri et al.
"Electronics Letters", vol. 24, No. 24, Nov. 24, 1988, pp. 1528-1529 by N.
Shibata et al.
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Primary Examiner: Chin; Wellington
Assistant Examiner: Negash; Kinfe-Michael
Attorney, Agent or Firm: Westman, Champlin & Kelly, P.A.
Claims
What is claimed is:
1. A wavelength division multiplexed optical fiber transmission equipment
for transmitting a plurality of signals having different wavelengths by
multiplexing the wavelengths, the equipment comprising a WDM optical
transmitting terminal including five or more optical transmitters for
outputting signals wherein the spacing between the wavelengths of two
first signals is re-used as the spacing between the wavelengths of two
other signals separated from said first signals by two or more signal
wavelengths and there is no periodicity in channel spacings.
2. A wavelength division multiplexed optical fiber transmission equipment
according to claim 1, wherein said WDM optical transmitting terminal
comprises polarization state control means for controlling the state of
polarization such that the states of polarizations of adjacent signals
cross each other at an output end of said WDM optical transmitting
terminal.
3. A wavelength division multiplexed optical fiber transmission equipment
according to claim 2, wherein said polarization state control means
comprises a first multiplexer for combining signals from optical
transmitters for odd numbered channels of the WDM optical transmitting
terminal, a second multiplexer for combining signals from even numbered
channels of the WDM optical transmitting terminal, and a polarization beam
combiner for controlling the state of polarization such that the states of
polarizations of signals from said first and second multiplexers cross
each other.
4. A wavelength division multiplexed optical fiber transmission equipment
according to claim 1, wherein said WDM optical transmitting terminal
comprises polarization state control means for controlling the state of
polarization such that, when any two waves having wavelengths are
selected, the states of polarizations of the two signals do not match each
other at an output end of said WDM optical transmitting terminal.
5. A wavelength division multiplexed optical fiber transmission equipment
according to claim 4, wherein said polarization state control means
comprises a plurality of polarization state control units for polarizing
signals from optical transmitters for respective channels at a
predetermined angle and a multiplexer for combining signals from the
plurality of polarization state control units.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a wavelength division multiplexed optical fiber
transmission equipment. More particularly, it relates to a wavelength
division multiplexed optical fiber transmission equipment which is capable
of increasing the transmission capacity of an optical fiber transmission
system by suppressing interference between codes caused by four-wave
mixing.
2. Description of the Related Art
Since an optical fiber transmission system which makes use of a wavelength
division multiplexed signal (to be abbreviated as WDM signal hereinafter)
can increase the transmission capacity of a transmission line without
making modifications on the transmission channel, its technology is
expected to be applied in a future optical fiber transmission system for
trunk lines. When a WDM signal is used, if four-wave mixing between signal
wavelengths is existent, four-wave mixing generated from two signal lights
adjacent to a signal light causes an inter-symbol interference in the case
of equal spacings between signal wavelengths. Thereby, the characteristics
of the system such as the maximum transmission distance and the maximum
number of channels of the WDM signal are limited. As a technology for
preventing deterioration in characteristics caused by this four-wave
mixing, there is known one in which the spacing between arbitrary two
wavelengths is made unequal (F. Forghieri et al., IEEE Photon. Technol.
Lett. vol. 6, no. 6, pp. 754-756).
An example of the technology is described with reference to FIG. 10. FIG.
10 shows the wavelengths of signal lights allocated to respective eight
channels as the axis of abscissas. As evident from the figure, the channel
spacing between first and second channels is 0.8 nm, the channel spacing
between second and third channels is 0.9 nm, the channel spacing between
third and fourth channels is 1.2 nm, and so on. In this way, the spacing
between arbitrary two chanells is made unequal to the spacing between any
other two channels.
By making the spacings unequal, it is possible to effectively suppress an
inter-symbol interference caused by four-wave mixing generated by signal
lights adjacent to another signal light.
However, when the spacings between respective two wavelengths are made
unequal, a required bandwidth of an optical signal becomes wider than when
the spacings between respective two wavelengths are equal. As the result,
the number of wavelengths able to be multiplexed is reduced. The reason
for this is as follows. Since the minimum channel spacing of a WDM signal
is limited by the performance of an optical de-multiplexer used for the
separation of the WDM signal as a matter of course, when x represents the
minimum channel spacing, a bandwidth nx is sufficient for a system in
which an n number of wavelengths are multiplexed with equal spacings while
a bandwidth for multiplexing an n number of wavelengths is much larger
than nx in a system with unequal spacings in which the spacings between
respective two wavelengths must be set larger than x.
In an example of the system shown in FIG. 10, since the minimum channel
spacing x is 0.7 nm, when the spacings between respective two wavelengths
are made equal, the bandwidth of optical signals for channels 1 to 8 is
4.9 nm. In contrast to this, when the spacings between arbitrary two
wavelengths are made unequal, the bandwidth needs to be 7 nm, about 1.4
times wider than when the spacings between respective two wavelengths are
made equal as understood from the figure.
Apart from a technology for making unequal the spacings between signal
wavelengths, there is known a technology for intentionally arranging
adjacent optical signals such that the states of polarizations of these
signals cross each other, making use of the fact that the generation
efficiency of four-wave mixing is high when the states of polarizations of
optical signals associated with the generation of four-wave mixing match
with each other and low when the states cross each other.
However, this technology is unsatisfactory as a technology for suppressing
generation of four-wave mixing when it is applied in a long-distance
wavelength division multiplexed transmission system because the states of
polarizations of adjacent signals cannot be maintained due to
birefringence of an optical fiber after the signals are transmitted
through some distance. Furthermore, when the states of polarizations of
adjacent optical signals are caused to cross each other, the states of
polarizations of signal wavelengths with one wavelength interposed
therebetween match each other. As the result, the generation efficiency of
four-wave mixing is increased.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a wavelength
division multiplexed optical fiber transmission system which can eliminate
the above problems of the prior art, suppress an inter-symbol interference
caused by four-wave mixing, and increase the number of wavelengths able to
be multiplexed while reducing a required bandwidth of optical signals.
To attain the above object, the present invention is characterized in that
a wavelength division multiplexed optical fiber transmission equipment for
transmitting a plurality of signals having different wavelengths by
multiplexing wavelengths comprises a WDM optical transmitting terminal
including 5 or more optical transmitters for outputting signals wherein
the spacing between the wavelengths of two signals is re-used as the
spacing between the wavelengths of two other signals separated from the
above signals by two or more waves and there is no periodicity in the
channel spacing s.
Further, the present invention is also characterized in that the WDM
optical transmitting terminal comprises polarization state control means
for controlling the state of polarization such that the states of
polarizations of adjacent signal wavelengths cross each other at an output
end of the WDM optical transmitting terminal.
The generation efficiency of four-wave mixing is in proportion to the light
intensify of each signal associated with the generation of four-wave
mixing and in inverse proportion to the spacing between the wavelengths of
signals (Reference Document: N. Shibata et al., Electronics Letters, vol.
24, pp. 1528-1529, 1988). Roughly speaking, when the channel spacing is
two times larger, the generation efficiency of four-wave mixing becomes
1/4. Therefore, expansion of the channel spacing is effective for
suppressing the generation of four-wave mixing. In a wavelength division
multiplexed optical fiber transmission system in which optical signals
having five or more wavelengths are multiplexed, when the spacing between
the wavelengths of two signals is re-used as the spacing between the
wavelengths of two other signals separated from the above signals by two
or more waves, four-wave mixing caused by the existence of two same
channel spacings can be suppressed by a large channel spacing obtained by
separating other optical signals by two or more waves. Therefore,
deterioration in the transmission characteristics of optical signals can
be effectively reduced. Further, since the bandwidth of the WDM signal can
be reduced by re-use of the spacing between wavelengths, the maximum
number of channels of the WDM signal can be increased.
When the states of polarizations of adjacent optical signals are caused to
cross each other at an output end of the WDM optical transmitting
terminal, generation of four-wave mixing between adjacent optical signals
can be suppressed for up to several hundred kilometers of a transmission
optical fiber adjacent to the transmitting terminal, thereby making
further improvement on the system.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a diagram showing an exemplary combination of signals to be
multiplexed according to a first embodiment of the present invention;
FIG. 2 is a block diagram of the system configuration of the first
embodiment of the present invention;
FIG. 3 is a block diagram of the configuration of an optical transmitter of
the first embodiment;
FIG. 4 is a block diagram of the configuration of a WDM optical
transmitting terminal according to a second embodiment of the present
invention;
FIG. 5 is a diagram explaining the states of polarizations of optical
signals obtained in the second embodiment;
FIG. 6 is a diagram showing an exemplary combination of signals to be
multiplexed of the prior art;
FIG. 7 is a diagram of measured bit error rate characteristic of each
channel for explaining the results of experiments based on the second
embodiment;
FIG. 8 is a block diagram of the configuration of a WDM optical
transmitting terminal according to the third embodiment of the present
invention;
FIG. 9 is a diagram explaining the states of polarizations of optical
signals obtained in the third embodiment; and
FIG. 10 is a diagram showing an exemplary combination of signals to be
multiplexed of the prior art.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is described in detail hereinunder with reference to
the accompanying drawings. The configuration of a first embodiment of the
present invention is first described with reference to FIG. 2.
The WDM optical transmitting terminal T is composed of optical transmitter
1 to 8 for channels 1 to 8 and a multiplexer 9. Meanwhile, a WDM optical
receiving terminal R is composed of optical receivers 11 to 18 for
channels 1 to 8 and a de-multiplexer 19. Although eight optical
transmitter and eight optical receivers are provided in this embodiment,
the present invention is not limited to this. Any terminal which includes
five or more optical transmitters or receivers is acceptable. The WDM
optical transmitting terminal T and the WDM optical receiving terminal R
are interconnected by a transmission optical fiber 21 into which optical
amplifiers 22 are inserted at a predetermined interval.
FIG. 3 is a block diagram of an exemplary optical transmitter 1. The
optical transmitter 1 is composed of a light source 1a, an external
modulator 1b, and an output terminal 1c. The light source la is composed
of a light emission unit, a drive current control unit, and a temperature
control unit. An electric current and temperature for driving the light
emission unit are stabilized at a high accuracy by the drive current
control unit and the temperature control unit. For instance, drive current
is stabilized at 10 .mu.A and temperature at 0.01.degree. C. The optical
transmitters 2 to 8 are composed likewise.
According to this embodiment of the present invention, for wavelengths of
signal lights output from the optical transmitter 1 to 8, the spacing
between the wavelengths of two signals is re-used as the spacing between
the wavelengths of other two signals separated from the above signals by
two or more waves and it is determined that there is no periodicity in the
wavelength spacings. For instance, the channel spacing is determined such
as shown in FIG. 1. That is, the wavelengths of signal lights output from
the optical transmitters 1 to 8 (channels 1 to 8) are determined to be
1554.9, 1555.7, 1556.6, 1557.7, 1558.5, 1559.5, 1560.7 and 1561.6 nm,
respectively.
As evident from FIG. 1, the spacing between channels 1 and 2 and the
spacing between channels 4 and 5 which are separated from channel 2 by two
channels are set to 0.8 nm. Also, the spacing between channels 2 and 3 and
the spacings between channels 7 and 8 separated from channel 3 by four
channels are set to 0.9 nm. There is no periodicity in the spacing between
channels.
Signal lights having respective wavelengths from the optical transmitters 1
to 8 (see FIG. 2) are combined by the multiplexer 9 and the combined light
is transmitted over the optical fiber 21 to the WDM optical receiving
terminal R. The signal light is transmitted while it is attenuated in the
transmission optical fiber 21 and amplified by the optical amplifiers 22
repeatedly. When the WDM optical receiving terminal R receives the signal
light, the signal light is divided by the de-multiplexer 19 and the
divided signal lights are transmitted to the optical receivers 11 to 18.
According to this embodiment of the present invention, the wavelengths of
signal lights from the optical transmitters 1 to 8 are set as shown in
FIG. 1 and four-wave mixing caused by the existence of two equal channel
spacing s can be suppressed by a large channel spacing obtained by
separating other light signals by two or more waves. Therefore,
deterioration in the transmission characteristics of optical signals can
be reduced effectively. As evident from comparison between FIG. 10 and
FIG. 1, the bandwidth of optical signals for channels 1 to 8 of this
embodiment is 6.7 nm (from 1561.6 to 1554.9) which is narrower than 7 nm
of FIG. 10. As the result, it is possible to use effectively the
amplifying bandwidth of the optical amplifier 22. In other words, the
maximum number of channels of the WDM signal can be increased.
A description is subsequently given of a second embodiment of the present
invention. FIG. 4 shows the configuration of the WDM optical transmitting
terminal T of this embodiment. Since other elements are the same or
equivalent as those of FIG. 2, they are not shown. A multiplexer 31 for
odd numbered channels combines signal lights from optical transmitters 1,
3, 5 and 7 for channels 1, 3, 5 and 7. A multiplexer 32 for even numbered
channels combines signal lights from optical transmitters 2, 4, 6 and 8
for channels 2, 4, 6 and 8. The combination of the states of polarizations
of signal lights for channels obtained by a polarization beam combiner 33
is shown in FIG. 5. That is, the states of polarizations of signal lights
for odd numbered channels and the states of polarizations of signal lights
for even numbered channels cross each other.
A description is subsequently given of the operation of this embodiment.
The wavelengths of signal lights from the optical transmitters 1 to 8 for
channels 1 to 8 are such as in the first embodiment that the spacing
between the wavelengths of two signals is re-used as the spacing between
the wavelengths of two other signals separated from the above signals by
two or more waves and it is determined that there is no periodicity in the
channel spacing s. Signal lights from the optical transmitter 1, 3, 5 and
7 for odd numbered channels are combined by the multiplexer 31 while
signal lights from the optical transmitters 2, 4, 6 and 8 for even
numbered channels are combined by the multiplexer 32. The combined signal
lights are transmitted to the polarization beam combiner 33 which causes
the states of polarizations of odd numbered channels and the states of
polarizations of even numbered channels to cross each other for output to
the transmission optical fiber 21. As the result, the states of
polarizations of adjacent channels cross each other at the transmitting
terminal.
Generally speaking, four-wave mixing does not occur between signal lights
whose state of polarization is cross each other. Therefore, according to
this embodiment, four-wave mixing generated between adjacent channels can
be suppressed. When the signal wavelength of each channel is determined as
shown in FIG. 1, four-wave mixing generated between even numbered channels
and between odd numbered channels which are the same in the state of
polarization does not cause interference to other channels. Therefore, the
effect of four-wave mixing generated every other channel can be
suppressed.
In contrast to this, in the prior art system in which the spacings between
wavelengths to be multiplexed are made equal (see FIG. 6), four-wave
mixing generated between odd numbered channels and between even numbered
channels which are the same in the state of polarization causes
interference to other channels, resulting in deterioration in
characteristics. The distance at which crossing between the states of
polarizations of signals is maintained is determined by the size of
birefringence of the transmission optical fiber and the spacing between
wavelengths to be multiplexed.
Experiments have been actually conducted with a combination of signal
wavelengths shown in FIG. 1 and equal spacings between wavelengths shown
in FIG. 6 to verify the effect of this embodiment. 66 km of the
transmission optical fiber was used in each section and 15 optical
amplifiers were used to construct a 1,000 km long experimental
transmission line. Since the optical bandwidth which can set the
wavelength of each signal is limited by a reduction in the bandwidth
caused by multi-relay by means of optical amplifiers, the maximum
bandwidth is about 9 nm in this embodiment, resulting in combinations of
signal wavelengths as shown in FIG. 1 and FIG. 6.
When a combination of signals as shown in FIG. 6 was used, even if
sufficient input power, i.e., -15 dBm, was provided to the optical
receivers, only a bit error rate of 10.sup.-6 could be attained for a
channel showing the best characteristics after transmission to a distance
of 1,000 km. In contrast to this, when a combination of signals as shown
in FIG. 1 was used, a digital error rate characteristic as shown in FIG. 7
was obtained. As understood from this figure, a good bit error rate of
10.sup.-9 or less was attained for all the channels after transmission to
a distance of 1,000 km with a relatively weak input power for optical
receivers of -30 to -25 dBm. Thus it was verified that the effect of the
embodiment is remarkable.
A description is subsequently given of a third embodiment of the invention.
FIG. 8 shows the configuration of a WDM optical transmitting terminal T of
this embodiment. The WDM optical transmitting terminal T is composed of
optical transmitters 1 to 8 for channels 1 to 8, polarization state
controllers 41 to 48 for channels 1 to 8, and a multiplexer 49. The
polarization state controllers 41 to 48 make settings so that, when any
two signal wavelengths are selected from channels 1 to 8, the states of
polarizations of the two signals do not match each other.
FIG. 9 shows an exemplary combination of the states of polarizations of
channels of this embodiment. In the case of the combination of the states
of polarizations of signals as shown in FIG. 9, unlike the second
embodiment, four-wave mixing is generated between the wavelengths of any
signals. However, according to this embodiment, the four-wave mixing is
not so large as four-wave mixing generated between odd numbered channel
and between even numbered channels in the second embodiment. Therefore,
the characteristics are equalized for all the channels and the
characteristics of the entire system are improved.
As is obvious from the above description, according to the present
invention, a plurality of optical transmitters output signals in which the
spacing between wavelengths of two signals is re-used as the spacing
between the wavelengths of other two signals separated from the above
signals by two or-more waves and there is no periodicity in the channel
spacing s. Therefore, deterioration in the transmission characteristics of
optical signals can be reduced effectively and the maximum number of
channels of the WDM signal can be increased.
In addition, when the states of polarizations of adjacent optical signals
are caused to cross each other at an output end of the WDM optical
transmitting terminal, generation of four-wave mixing between adjacent
optical signals can be suppressed for up to several hundred kilometers of
a transmission optical fiber adjacent to the transmitting terminal,
thereby further improving the transmission characteristics of an optical
fiber transmission system.
Since a plurality of optical transmitters control the state of polarization
such that, even when any two waves having different wavelengths are taken
out, the states of polarizations of the two signals do not match each
other at an output end of the WDM optical transmitting terminal, the
transmission characteristics of all the channels are equalized and as the
result, the transmission characteristics of the entire optical fiber
transmission system can be improved.
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